Depression is associated with a significantly, increased risk for morbidity and mortality in patients with coronary artery disease (CAD), but the mechanisms underlying this association remain speculative. The present uncertainty about specific mechanisms makes it difficult to undertake appropriate assessment or provide the most effective treatment. The two most likely mechanisms are increased ischemic activity and/or decreased adherence to the medical treatment regimen. The primary objective of the proposed research is to evaluate the relationship of depression to these potential mechanisms. Four hundred fifty patients undergoing thallium exercise testing who are referred for elective cardiac catheterization and angiography and who are subsequently found to have significant coronary artery disease (50% or greater stenosis in one or more major coronary artery or branch) will be recruited. They will receive a structured psychiatric diagnostic interview, psychological tests, and a standard 24 hour ambulatory ECG. Depressed and non-depressed patients will be compared with respect to specific manifestations and extent of ischemia, including silent and symptomatic, as determined by thallium stress testing and 24 hour Holter monitoring. Heart rates, heart rate variability, and the incidence and complexity of ventricular ectopic activity will also be compared. In addition, the patients will be asked to allow the hospital pharmacy to fill their cardiac related prescriptions upon discharge and utilize an electronic pill monitor which records the time each pill is removed. Adherence frequency and patterning will be compared between depressed and non-depressed patients for the first 42 days of drug treatment following their hospital discharge. Finally, depressed and non-depressed patients will be compared on a number of demographic and medical parameters including the presence of secondary medical illnesses and major risk factors.